Figure 1. A High-Contrast Scene cannot readily be imaged in full detail on a CCD. Depending on the optical and electronic camera settings, the image tends to be either overexposed in the bright areas or underexposed in the dark areas.

A simple technique for extending the dynamic range of a charge-coupled-device (CCD) video camera involves the use of photochromic material — the same material used in self-adjusting sunglasses. The dynamic range of an image is the ratio between the maximum and minimum brightness levels in the image. The dynamic range of a CCD is the ratio between an overexposure brightness level (above which the image becomes saturated or "washed out") and an underexposure level (below which details disappear into the darkness). The dynamic range of a CCD is less than that of the human eye; for example, a human observer can often see both shadowed and unshadowed features in a scene illuminated by sunlight, whereas a CCD cannot capture details simultaneously in both the brightest and darkest parts of the scene (see Figure 1).

The present technique provides for compression of the dynamic range of brightness of an image focused on a CCD so that all or most parts of the image lie within the dynamic range of the CCD. When such compression is effected, the CCD output can be expected to show details in both the brightest and darkest parts of the scene.

Figure 2. A Photochromic Filter at the Image Plane becomes, in effect, a temporary photographic negative with a dynamic range somewhat less than that of the image. Therefore, the net image projected through the filter looks like the original scene, except that it has less dynamic range.

In practice, compression of the dynamic range of brightness in an image must be accomplished through local darkening of the image, with greater darkening in brighter locations. Photochromic material exhibits the required greater darkening with exposure to brighter light. In the present technique, a photochromic filter is placed at (or immediately in front of) the CCD image plane, so that it becomes darkened in the bright areas of the image (see Figure 2). As a result, the light that passes through the photochromic filter forms the desired reduced-dynamic-range version of the image.

This work was done by Richard A. Volpe of Caltech for NASA's Jet Propulsion Laboratory. NPO-20254

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Photochromic image-plane filter extends dynamic range of CCD

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This article first appeared in the August, 1998 issue of Photonics Tech Briefs Magazine.

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